(1) Field of the Invention
The present invention relates to a mixed dual frequency generating system for outputting a digital signal having, when converted into an analog signal, a desired frequency or frequencies. In particular, it relates to a mixed dual frequency generating system for providing a digital frequency-shift keying (FSK) signal or for providing a digital signal having, when converted into an analog signal, two mixed frequencies.
(2) Description of the Related Art
The transmission of analog FSK signal is disclosed in, for example, U.S. Pat. No. 4,368,439. In the field of telephone-line systems, however, wide use is made of PCM channels through which a so-called digital FSK signal is transmitted. The digital FSK signal consists of a sequence of binary coded signals expressing a logic "1" and a logic "0". Each of the binary coded signals in the digital FSK signal consists of a PCM signal obtained by sampling a sine wave. That is, the logic "1" in the digital FSK signal is formed by a PCM signal having, when converted into an analog signal, a first frequency, and the logic "0" in the digital FSK signal is formed by a PCM signal having, when converted into an analog signal, a second frequency, different from the first frequency. For example, according to CCITT, V21, the two frequencies are 980 Hz and 1180 Hz, respectively.
In a conventional digital FSK signal generating device, two PCM signal sources are employed for generating the above-mentioned two PCM signals. These two PCM signals are always generated from the two PCM signal sources, and a selection unit switches from one of the two PCM signals to the other PCM signal to be output from the device.
There are, however, disadvantages in the above conventional digital FSK signal generating device. That is, it needs two independent PCM signal sources, making it expensive, and at a switching point, these two PCM signals are not always in the same phase. Therefore, at the switching point, the phases of the PCM signals, or analog signals obtained from the PCM signal, usually become discontinuous, as later described in more detail. Because of this discontinuity, the digital FSK signal at the switching point has an impulse noise and, therefore, may be erroneously received at a receiver as noise or as another error signal.
Apart from the digital FSK signal generating device, a device for generating two mixed signals having different frequencies is useful for various applications. For example, in an examination of a modem, by applying a signal having two mixed frequencies, the characteristics for respective frequency components can be simultaneously examined. Also, a tone signal composed of two mixed frequencies is often used.
In order to generate such a signal having two mixed frequencies, a conventional technique is to provide two oscillators respectively generating frequencies different from each other. In this conventional case also, the need for two oscillators makes the device expensive.
A read only memory (ROM) may be used to obtain two signals having different frequencies by storing mixed data obtained by sampling two sine waves having different frequencies. By sequentially reading the ROM with a sampling period the mixed data can be read. In this case, however, it is impossible to change the frequencies of the output two signals because the stored mixed data are fixed data and are read step by step sequentially. Also, because the ROM must store mixed data for two different frequencies, the memory size of the ROM must be large.